This invention concerns a valve gasket and a process for its manufacture. More particularly, it concerns a gasket that resists high temperatures and pressures designed for a rotary gate valve.
Generally, an elastic gasket, which may be made of an elastomer, is used to form the seal between a valve and its pivoting gate. Gaskets of this type perform this function properly when the temperature of the fluid flowing through the valve does not exceed 120.degree. C., when the pressure is less than about 20 bars, and when the fluid does not degrade the material from which the gasket is made.
When the conditions of use become more severe as a result of higher temperatures, high pressures, and greater corrosiveness of the liquid being carried, however, elastomer gaskets are no longer suitable. In such cases other materials are used, such as fluorocarbon polymers, of which the best known is polytetrafluoroethylene (PTFE). These materials are inert with respect to many fluids and can also withstand much higher pressures and temperatures than elastomeric materials. However, such fluorocarbon polymers have several drawbacks. After they have been deformed they regain their initial shape only very slowly, and when they are subjected to prolonged deformation, either through compression or traction, they become quasipermanently deformed, a phenomenon known by the name of gasket creep. As a result, when a gasket is compressed for a long period between the valve body and the gate, it gradually becomes deformed and no longer provides a proper seal. A known method of preventing this phenomenon is to surround the gasket with one or more elastic rings that prevent radial creep. Reinforced in this way, the gasket produces a satisfactory seal. However, when the fluid flowing through the valve is corrosive to ordinary metals, the elastic rings reinforcing the gasket must be made of corrosion-resistant noble metals. This greatly increases the cost of producing the gasket.
One way of getting around this drawback consists of embedding the elastic rings within the gasket material, e.g., by double molding. This known technique is easy to use when the gasket is made of an elastomer, since elastomers mold easily. On the other hand, the same technique cannot be used when the gasket is made of pure PTFE or is mixed with a filler such as carbon powder or stainless steel powder. Producing a PTFE part involves a process of sintering that consists of compacting a powder in a mold under very high pressure, followed by the heating of the molded part to melt together the grains of compacted powder. As a result of the special technique required for the production of PTFE parts the double molding of an elastic ring inside a gasket presents almost insurmountable difficulties since, when the powder is compacted, the ring moves and its position becomes unknown. Furthermore, during the heating the gasket and ring become deformed in an uncontrolled manner, since the expansion coefficient of PTFE is about ten times higher than that of the material making up the ring.